Diabetes mellitus is a chronic disorder characterized by insulin deficiency, hyperglycemia and high risk for development of complications of the eyes, kidneys, peripheral nerves, heart and blood vessels. The disease is highly prevalent, affecting approximately 30 million people in the U.S. Monitoring of glycemic control has traditionally been considered a cornerstone of diabetes care. The importance of monitoring glycemia has been established by studies proving a direct relationship between mean blood glucose and the development and progression of the chronic complications of diabetes. The landmark Diabetes Control and Complications Trial (DCCT), completed in 1993, showed that risks for development and/or progression of the chronic complications of type 1 diabetes is closely related to the degree of glycemic control, as assessed by HbA1c determinations. The importance of HbA1c as a marker of glycemic control in diabetes mellitus, and more recently as a diagnostic tool, is now well established. This proposed project will improve the measurement of HbA1c for optimal clinical and diagnostic use. This will be done through an established standardization program by continuously tightening criteria for method and laboratory certification, monitoring performance of laboratory methods and laboratories, and testing and publishing data on HbA1c analytical and biological interferences. Preservation of beta cell function in type 1 diabetes has been identified as an important goal in delaying progression of the disease at onset and also for therapeutic intervention. Accurate, standardized measurement of C-peptide will aid in this goal and is also the subject of this proposal. Standardization will be accomplished by implementation of a traceability scheme with re-calibration of C-peptide methods by each manufacturer and monitoring the effects of re-calibration by evaluation of proficiency testing data. In addition, criteria will be formulated for acceptable accuracy, precision and specificity of C-peptide assays.